Monoclonal antibodies have many applications as research tools and, increasingly, as therapeutic or diagnostic agents. Currently more than 20 different monoclonal antibodies have received regulatory approval to treat a variety of different diseases, including cancer, inflammation, auto-immune disorders, infectious disease, asthma, cardiovascular diseases and transplant rejection and the number of monoclonal antibody drugs in the development pipeline is increasing year-on-year.
The utility of rodent (specifically murine) monoclonal antibodies in human therapy is limited because of problems associated with their non-human origin, in particular their immunogenicity in a human host. In order to minimize the human immune response against therapeutic antibody drugs, monoclonal antibody technology has evolved from full mouse antibodies to chimeric antibodies (mouse variable domains grafted on a human IgG backbone), to humanized antibodies (mouse CDRs grafted on a human IgG backbone), to “fully human” antibodies derived from synthetic and non-immune libraries or immunized transgenic mice expressing part of the human IgG repertoire.
A number of technology platforms have been developed which allow production of fully human or “humanized” monoclonal antibodies against target antigens of therapeutic interest. Each of these platforms has its own particular characteristics and potential shortcomings.
Humanisation of mouse monoclonal antibodies was initially achieved by combining mouse variable domains with human constant domains, creating so called chimeric antibodies having about 70% of human content. A further degree of humanization was subsequently achieved by grafting the complementarity-determining regions (CDRs) of mouse monoclonal antibodies onto human framework regions of the variable antibody domains of human antibodies. In addition, several amino acid residues present in those framework regions were identified as interacting with the CDRs or antigen and were back mutated in the humanized antibody to improve binding. (Almagro et al. Frontiers in Bioscience. 13: 1619-1633 (2008)). Monoclonal antibodies engineered using this approach have a relatively high degree of primary sequence homology to human VH and VL domain sequences after humanisation, but a drawback is the possibility of ending up with hypervariable loops not having human-like structure, because not all mouse-encoded CDRs use canonical folds, and canonical fold combinations, which are not found in human antibodies (Almagro et al., Mol. Immunol. 34:1199-1214 (1997); Almagro et al., Immunogen. 47:355-63 (1998)). A further drawback is the large number of mutations typically required to humanise such antibodies (the procedure for which is complex and time-consuming), with the consequent risk of losing affinity and potency as a result of the number of changes needed for humanisation and, the fact that VKappa domains are mainly used in the murine repertoire, whereas approximately half of all human antibodies possess VLambda domains. Alternative humanization procedures include resurfacing or veneering, in which only solvent exposed framework residue deviating from human are substituted, while buried or partly buried residues and residues involved in interdomain contacts are maintained (Padlan E. (1991) Mol. Immunol.). A more stringent method is SDR grafting, in which CDR grafting onto human FRs is applied, but on top of this residues within the CDRs are mutated to the human counterpart maintaining only the Specificity Determining Residues (SDRs) which are contacting the antigen (Kashmiri S. V. S. et al. (2005) Methods).
As a potential improvement on humanised mouse monoclonal antibodies, “fully human” monoclonal antibodies can be produced by two very different approaches. The first approach is selection from a fully synthetic human combinatorial antibody library (for example HuCAL®, MorphoSys) or from a non-immune antibody library (Vaughan T. J. et al (1996) Nat. Biotechnol., Cambridge Antibody Technology; de Haard H. J. et al (1999) J Biol. Chem., DYAX). The potential drawback of this approach is that the such libraries only approximates the functional diversity naturally present in the human germline, thus the diversity is somewhat limited. Also, antibodies generated using this approach do not contain in vivo selected CDRs as these occur in antibodies obtained via active immunisation, and typically affinity maturation has to be done in order to improve affinity for the target antigen. Affinity maturation is a lengthy process which may add considerable time to the antibody discovery process. Also, in the process of affinity maturation certain amino acid residues may be changed which may negatively affect the binding specificity or stability and production of the resulting antibody (Wu et al., J. Mol. Biol. 368: 652-65 (2007)).
Alternative “fully human” platforms are based on transgenic mice which have been engineered to replace the murine immunoglobulin encoding region with antibody-encoding sequences from the human germline (for example HuMab, Medarex). These systems have the advantage that antibodies are raised by active immunisation, with the target antigen, i.e. they have a high starting affinity for the antigen, and that no or only minimal antibody engineering of the original antibodies is required in order to make them more human-like. However, the transgenic mouse strains are by definition highly inbred and this has adverse consequences for the strength and diversity of the antibody response. Another drawback with this platform may be impaired B cell maturation due to human Fc/mouse Fc receptor interaction in some transgenic mouse systems.
A further platform is based on immunisation of non-human primates, specifically cynomologous monkeys. Due to the high degree of amino acid sequence identity between monkey and human immunoglobulins it is postulated that antibodies raised in monkeys will require little or no additional “humanisation” in the variable domains in order to render them useful as human therapeutics (see WO 93/02108). However, quite often non-human canonical fold combinations for CDR1 and CDR2 in the VH are observed in these primatized antibodies.